Many semiconductor imaging sensors today are front side illuminated. That is, they include imaging arrays that are fabricated on the front side of a semiconductor wafer, where light is received at the imaging array from the same front side. However, front side illuminated imaging sensors have many drawbacks, one of which is a limited fill factor.
Backside illuminated imaging sensors are an alternative to front side illuminated imaging sensors that address the fill factor problems associated with front side illumination. Backside illuminated imaging sensors include arrays of pixels that are fabricated on the front surface of the semiconductor wafer, but receive light through a back surface of the wafer. Color filters and micro-lenses may be included on the back surface of the wafer in order to improve the sensitivity of the backside illuminated sensor. However, the sensitivity of a pixel in the array varies depending on its location within the array. For example, a pixel located at or near the center of the array may have a high sensitivity due to incident light near the center of the array being normal to the back surface of the sensor. While pixels at or near the center of the array may have a high sensitivity, pixels that are located away from the center of the array may have a lower sensitivity due to the titled angle of the incident light relative to the back surface of the sensor. This variance in sensitivities across the pixel array is often referred to as “sensitivity roll off” and may result in irregular white balance and poor color uniformity of the image sensor.